A single drop of the hard stuff? (9)
I believe the answer is:
hailstone
'a single' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'drop of the hard stuff?' is the wordplay.
'drop of' becomes 'hail' (I can't explain this - if you can you should give a lot more credence to this answer).
'the hard stuff?' becomes 'stone' (I've seen this in another clue).
'hail'+'stone'='HAILSTONE'
Can you help me to learn more?
(Other definitions for hailstone that I've seen before include "It lands with a noise" , "Small pellet of ice" , "Frozen pellet" , "a drop of the hard stuff?" , "Hard precipitation" .)